This invention relates generally to signal processing techniques for fiber optic sensor systems. This invention relates particularly to demodulation of signals output from an array of fiber optic interferometric sensors for determining changes in a physical parameter measured by the individual sensors.
Mismatched fiber optic interferometers are commonly used as sensing elements in fiber optic sensor arrays for measuring changes in a parameter such as fluid pressure, acceleration, magnetic field intensity, etc. Such sensing elements measure the time-varying phase delay between two optical paths having unequal path length. Typically in time division multiplexed (TDM) systems, a modulated optical signal is input to the sensor array, and various demodulation techniques have been proposed for correlating signals output from the array with the sensors that produced the signals.
Common to all demodulation methods for fiber optic interferometric sensor arrays is the acquisition of an in-phase term proportional to the cosine of the interferometer phase shift and a quadrature term proportional to the sine of the interferometer phase shift. The sine of the sensor phase shift is referred to as the quadrature term, Q; and the cosine of the sensor phase shift is referred to as the in-phase term, I. The angle of the phase shift is determined by calculating ratio Q/I, which is the tangent of the sensor phase shift. The amplitudes of the sine and cosine terms must be set equal by a normalization procedure to ensure the successful implementation of an arctangent routine to find the sensor phase shift.